1. Field
The disclosed concept relates generally to electrical switching apparatus and, more particularly, to electrical switching apparatus, such as circuit breakers. The disclosed concept also relates to opening assemblies for electrical switching apparatus.
2. Background Information
Electrical switching apparatus, such as circuit breakers, provide protection for electrical systems from electrical fault conditions such as, for example, current overloads, short circuits, abnormal voltage and other fault conditions. Typically, circuit breakers include an operating mechanism, which opens electrical contact assemblies to interrupt the flow of current through the conductors of an electrical system in response to such fault conditions as detected, for example, by a trip unit. The electrical contact assemblies include stationary electrical contacts and corresponding movable electrical contacts that are separable from the stationary electrical contacts.
FIGS. 1A and 1B, for example, show a portion of a power air circuit breaker 1. The power air circuit breaker 1 uses opening springs 3 (one opening spring 3 is shown in simplified form in FIGS. 1 and 2) to achieve and maintain full opening gap (e.g., separation of the electrical contacts) during opening and, in some cases, to augment the opening speed to improve interruption. In order to minimize the required closing energy, the minimum possible opening spring force and energy is desired. Each opening spring 3 is attached at its moving end to an arm 5, which is fixed to the poleshaft 7. This arrangement stretches the spring 3 from open length, Lo (FIG. 1A) to closed length, Lc (FIG. 1B) as the poleshaft 7 rotates from open (FIG. 1A) to closed (FIG. 1B). The poleshaft 7 is commonly designed to maintain a substantially constant moment arm (see, for example, open moment arm, Mo of FIG. 1A and closed moment arm, Mc of FIG. 1B).
Achieving and maintaining full opening gap becomes especially difficult after interruption, when debris and shunt behavior cause the opening force requirement to increase. One option is to strengthen the opening springs. However, strengthening the opening springs without a corresponding increase in closing springs may lead to stalling and incomplete closures. Also, increased spring forces result in greater frictional forces that tend to resist desired movements of the circuit breaker. The difficulty of closing against stronger opening springs is more pronounced late in closing, once the moving contacts seat on the stationary contacts and the contact springs become a contributing factor. Increasing the closing springs to overcome stronger opening springs also adds cost, reduces life, and increases the requirements of some accessories such as, for example and without limitation, the closing solenoid and the charging motor. The foregoing difficulties become progressively more problematic as additional circuit breaker poles are added.
There is, therefore, room for improvement in electrical switching apparatus, such as circuit breakers, and in opening assemblies therefor.